Iridescent ammonite fossil. Shown with a quarter to illustrate size.
Ammonite, which means coiled horn, was a prehistoric air breathing invertebrate. It looked like an ancestor of the squid except its body was protected by a shell which it grew by adding chambers in a spiral pattern. Ammonites are an extinct group of marine animals in the Cephalopoda class. The iridescent colours are caused by the interference with the light that reflects back from thin layers of aragonite and conchiolin built up in the animal during its life cycle, then preserved through fossilization.